Origin

The Four Parts

The four parts are called Tenor, Lead, Baritone and Bass. The melody is usually sung by the lead because he takes the lead and the others "follow", or "fit in" with this lead.

The Tenor harmonizes above the melody. In the men's groups the tenor is sung in falsetto to avoid drowning out the lead. The listener's ear normally hears the highest note most easily, so that is why normally male and female groups are separate. Some mixed groups exist, but it is very difficult to prevent the high female voices drowning out the lower male voices. 

The Bass sings the lowest harmonizing notes below the melody. The base forms the foundation for the chord structure. 

The Baritone completes the chord, usually below the melody, but can be above it for a few bars at a time. 

The melody may be sung occasionally by the Bass, but not by the tenor except for an infrequent few notes to avoid awkward voice leading, and in introductions or tags.

The Chord Structure

Major and minor chords are used, and at least a third of the chords should be dominant seventh chords, resolving primarily on the circle of fifths. Sixth, ninth and major seventh chords are avoided except where demanded by the melody, while chords containing the minor second interval are not used at all. The basic harmonization may be embellished with additional chord progressions to provide harmonic interest and rhythmic momentum, to carry over between phrases, or to introduce or close the song effectively. 

Pitch

The human voice has the advantage that the notes are not fixed pitch like a piano. The piano tuner sets the pitch of each note, to a compromise value. This means that the piano can never play a perfectly tuned chord - it is always a compromise chord. Barbershop singing tries to tune every chord perfectly in relation to the note sung by the Lead, each part adjusting to match the note sung by the lead. When this is successful a fifth note can be heard, that no-one is singing, soaring above the rest. This fifth note is formed by perfectly matched harmonics and the sound is called a "ringing chord". 

The Director

Barbershop music is always memorized. This means that every singer can watch the director, and has his or her hands free for gestures. The Director takes wide liberties with note values, and uses tempo and volume changes to create a mood and tell the story more effectively. 

Enhanced Sound

Amplification is not needed as a general rule. When everyone in a quartet matches pitch and vowel sound perfectly, they sound much louder than they are actually singing, and they can easily be heard by a hundred people. A larger chorus can fill an auditorium with sound, and amplification is only needed for the announcer who introduces the songs.